Catalyst

ABSTRACT

A catalyst composition useful for the oxidation of olefins, particularly the oxidation of propylene and isobutylene to produce the corresponding unsaturated aldehydes acrolein and methacrolein, respectively, comprises the combination of oxides of molybdenum, cobalt, iron, bismuth, thallium and antimony, and preferably also silicon. When the catalyst is used for the vapor-phase oxidation of the olefins with molecular oxygen, the aldehydes are produced with high selectivity. Alcohol precursors for the olefins can be used as feed instead of the olefins themselves.

This invention relates to catalysts, and is more particularly concernedwith catalysts for the vapor-phase oxidation with molecular oxygen oflower olefins to the corresponding unsaturated aldehydes of the samenumber of carbon atoms, and to a process for using such catalysts. Thecatalysts and process of this invention are particularly useful in theoxidation of propylene and isobutylene to acrolein and methacrolein,respectively.

It is well known that unsaturated aldehydes, such as acrolein andmethacrolein, can be produced by the vapor-phase oxidation of thecorresponding olefins by means of molecular oxygen in the presence of asuitable oxidation catalyst. A variety of catalyst compositions havebeen proposed for this purpose and many such compositions comprise theoxides of molybdenum, iron, bismuth, cobalt and/or nickel. As a generalrule, however, the selectivity to the desired aldehyde, i.e. the molarquantity of aldehyde obtained per mol of olefin converted, has beenrelatively low when catalyst compositions of this type have beenemployed. More recently, attempts have been made to increase theselectivity of the reaction by incorporating oxides of more unusualelements in the catalyst. For example, Shiraishi et al. U.S. Pat. No.3,928,462 proposes the inclusion of thallium oxide and, optionally, theoxides of various other elements. The resultant catalyst compositionmakes possible increased selectivity, but substantial amounts of theolefin reacted are still converted to undesired by-products, such ascarbon monoxide and carbon dioxide, so that the selectivity values stillleave substantial room for improvement.

It is, accordingly, an object of this invention to provide a novel andimproved catalyst composition which is effective in converting olefinsto the corresponding unsaturated aldehydes with surprisingly highselectivity.

It is a further object of the invention to provide a process forconverting olefins such as propylene and isobutylene to acrolein andmethacrolein, respectively, with minimum formation of undesiredbyproducts such as carbon monoxide and carbon dioxide.

Other objects of the invention will be apparent from the followingdetailed description of the catalyst composition and process whichcharacterize the invention.

It has been discovered that the desired conversion of olefins to thecorresponding unsaturated aldehydes with high selectivity can beeffected by carrying out the vapor-phase molecular oxidation of theolefins in the presence of a catalyst composition which comprises oxidesof molybdenum, cobalt, iron, bismuth, thallium and antimony, and siliconmay be optionally included. More specifically, the catalyst compositionof the invention comprises the oxides of the specified elements in thefollowing atomic ratios: Mo = 12, Co = 0.2-8, Fe = 0.05-5, Bi = 0.2-4,Tl = 0.05-5, Sb = 0.1-5, Si = 0-20. The catalyst composition may beregarded either as a mixture of oxides of the named elements or asoxygen-containing compounds of the elements. As prepared and/or underthe reaction conditions, the catalyst may contain either or both forms.The catalyst composition of the invention may then be expressed by thefollowing general formula:

    Mo.sub.a Co.sub.b Fe.sub.c Bi.sub.d Tl.sub.e Sb.sub.f Si.sub.g O.sub.h

wherein a to h indicate the atomic ratio of each component and a is 12,b is 0.2-8, c is 0.05-5, d is 0.2-4, e is 0.05-5, f is 0.1-5, g is 0-20and h has a value which is determined by the valence and proportions ofthe other elements in the catalyst.

The catalyst composition is preferably used in unsupported form, e.g. inthe form of pellets or other like compressed shapes of various sizes,the composition may be formed in conventional manner using techniqueswell known to persons skilled in the art. For example, compounds ofmolybdenum, cobalt, iron, thallium, antimony and bismuth are eachdissolved in a small amount of water or other solvent, and the solutionsare then combined and evaporated to dryness, e.g. in a rotary dryer. Toprepare the catalyst the several components can be introduced intosolution in the form of various salts or other compounds of convenienttypes and no specific form for the catalyst precursors is necessary. Theuse of ammonium salts, halides, e.g. chlorides, nitrates or acid formsof the elements to be supplied are, however, particularly suitable.Preferably, however, aqueous solutions are employed and water-solubleforms of the elements are used. In some cases the solutions may haveacids and/or bases added to them to facilitate dissolution of thecatalyst precursors. For example, acids such as hydrochloric or nitricor bases such as ammonium hydroxide can be used if desired. When siliconis to be employed as an optional component, it is suitably added in theform of an aqueous colloidal solution of SiO₂. The resulting powder fromthe evaporation is then thoroughly dried and preferably screened toeliminate large particles which make it difficult to produce uniformcompressed shapes, such as pellets. Typically, the powder is passedthrough a 20-mesh screen. The powder is then mixed with an organicbinder of any conventional type, such as polyvinyl alcohol, and themixture is thoroughly dried and again screened, typically to provide a20-80 mesh size. The dried mixture is then preferably combined with alubricant, again of any conventional type, such as stearic acid, andcompressed into the desired shape, e.g. pelletized, or extruded orotherwise shaped, the compressed shapes typically having heights anddiameters of 1/16 inch to 3/8 inch. Finally, the thus-produced catalystcomposition is activated at high temperature for a prolonged period inaccordance with conventional practice in this art. For example andtypically, the pellets are placed in an oven or kiln, or in a tubethrough which air is passed, at an elevated temperature (e.g. 300°-500°C, preferably 325°-450° C) for at least ten hours. In a particularlypreferred activation step, the temperature is raised at the rate of 20°C per hour to 400°-450° C and this temperature is maintained for 16hours.

It will be understood that the foregoing description regardingpreparation of the catalyst in a form suitable for use in a vapor-phaseoxidation reaction is merely illustrative of many possible preparativemethods and is given solely by way of exemplification. This method is,however, particularly suitable and is preferred.

When the catalyst of this invention is used in the vapor-phase oxidationof olefins to form the corresponding unsaturated aldehydes, theoxidation conditions employed are those generally associated with thisreaction. Thus, the reaction in which the catalyst compositions of thisinvention are of particular utility and in which they provide highselectivity involves contacting the appropriate olefins, e.g. propyleneor isobutylene in the vapor phase with the catalyst and oxygen,preferably also in the presence of steam. Once reaction is begun, it isself-sustaining because of its exothermic nature. A variety of reactorswill be found to be useful and multiple tube heat exchanger typereactors are quite satisfactory, and the process can be carried out inconventional equipment commonly employed for reactions of this type.

The gaseous feed to the reactor contains relatively low concentrationsof olefin, oxgyen and steam. Suitably, an inert gas, such as nitrogen,is also present. The oxygen is usually added as such or as air or as airenriched with oxygen. As mentioned, conventional oxidation conditionscan be employed but, for best results, the olefin is generally presentin concentrations of about 2 to 20 volume percent of the total feed witha preferred range of about 5 to 15 volume percent, and the correspondingranges for oxygen are 4 to 20 volume percent and 5 to 15 volume percentand for steam up to 30 volume percent and 5 to 25 volume percent, thebalance being the inert gas or gases.

The temperature of the reaction at the center of the reactor should, forbest results, be within the range of from about 330°0 to 500° C.,preferably 350°-400° C. and the optimum temperature range is 360° to370° C. Because the reaction is exothermic, means for conducting theheat away from the reactor are normally employed. The temperature may becontrolled by conventional methods such as by the use of reactorssurrounded by a salt bath.

The pressure in the reactor is not generally critical, and the reactionmay be conducted at atmospheric, superatmospheric or subatmosphericpressure. Preferably, however, pressures ranging from atmospheric up to200 psig, preferably up to 100 psig, and most preferably up to 75 psigare employed.

The catalyst and the processes of the present invention are useful forthe production of unsaturated aldehydes by oxidation with molecularoxygen of lower olefins generally. The preferred starting materials arethe monoethylenically unsaturated olefins of from 3 to 4 carbon atoms.Best results have been obtained with isobutylene. Mixtures of olefinsmay be used.

The unsaturated aldehyde product may be recovered by a number of wayswell known to those skilled in the art. For example, the aldehyde may becondensed, or scrubbed with water or other suitable solvents, followedby separation of the unsaturated aldehyde product from the scrubbingliquid. The gases remaining after the aldehyde-removal step are suitablyrecycled to the reaction, if desired, preferably after removal of net COand CO₂ by conventional means, e.g. absorption in aqueous sodiumhydroxide solution.

The features of the invention will be more readily apparent from thefollowing specific examples of typical application. It will beunderstood, however, that these examples are for the purpose ofillustration only and are not to be interpreted as limiting theinvention.

EXAMPLE I

In 200 cc of water are dissolved 106 grams of the molybdenum salt (NH₄)₆Mo₇ O₂₄ 4H₂ O. Then 58 grams of Co(NO₃)₂.6H₂ O are dissolved in 150 ccof water, 10.1 grams of Fe(NO₃)₃.9H₂ O are dissolved in 50 cc of water,13.3 grams of TlNO₃ are dissolved 100 cc of water, 11.4 grams of SbCl₃are dissolved in a mixture of 16 cc of water, 4 cc of concentrated HCland 25 cc of ammonium hydroxide, and 24.3 grams of Bi(NO₃)₃.5H₂ O aredissolved in a mixture of 10 cc of water, 10 cc of concentrated nitricacid and 50 cc of ammonium hydroxide. These solutions, together with 80grams of 30% collodial silicon dioxide, are fed to a rotary dryer of4000 cc capacity and the mixture in the dryer is evaporated to drynessat a temperature of 200° C. The resulting powder is removed from thedryer and dried in an oven at 150° C. for 18 hours. The dried powder isscreened through a 20-mesh screen, a 4% aqueous solution of polyvinylalcohol is added in sufficient quantity to make a damp mixture and thismixture is dried at 80°-90° C. for four hours. The dried mixture is thenscreened to 20-80 mesh, and about 5-8% of stearic acid powder isthoroughly mixed with it. The resulting mixture is then pelletized toform pellets of 3/16 inch height and diameter. The pellets are thenactivated in an oven by heating them gradually at a rate of 20° C. perhour to 400°-450° C. and maintaining them at this temperature for 16hours. The activated pellets have a density of 0.95 gm/cc and thecatalyst components molybdenum, cobalt, iron, thallium, antimony,bismuth and silicon are present in the atomic ratio of 12, 4, 0.5, 1, 1,1 and 8, respectively.

A 60 cc quantity of this catalyst composition is placed in a reactordefined by a 1/2 inch × 45 inches stainless steel pipe, the reactor pipebeing filled with 30 cc of inert filler (silicon carbide) below thecatalyst bed and 50 cc of the inert filler above the catalyst bed inconventional manner to insure uniform temperature contact with thecatalyst. Nitrogen-diluted mixtures containing methacrolein, oxygen andsteam in various proportions are fed to the reactor at a pressureslightly above atmospheric (2-4 psig), at temperatures ranging from 298°C.-308° C. and at a space velocity of about 1200 hr.⁻¹. The term "spacevelocity" is used in its conventional sense to mean liters of gas (STP)per liter of catalyst per hour. The reaction is run continuously withcontinuous feed and continuous withdrawal of exit gas but the exit gasis analyzed at intervals of several hours and the feed composition is inmost cases varied at these intervals to give the overall effect of aseries of different runs under differnt conditions. Analyses are carriedout by means of gas chromatography and by absorption of the CO₂ insodium hydroxide solution, using conventional techniques. The feedcomposition, the conditions of operation and the results of theseexperiments are set forth in the following tables, A and B. The olefin,oxygen and steam content of the feed is specified, the balance beingnitrogen, the olefin and oxygen being determined on a dry basis.Experiments 1-9 represent one continuous run with each experimentindicating the analysis of feed and exit gas at each indicated time inhours from the start of the run. Experiments 10 to 19 represent anothercontinuous run.

                  TABLE A                                                         ______________________________________                                        Exp. Temp.             Space Vel.                                                                             Feed, Vol. %                                  No.  ° C.                                                                           Time, hr. hr..sup.-1                                                                             C.sub.4 H.sub.8                                                                     O.sub.2                                                                            Steam                              ______________________________________                                         1   370     280       3200     5.5   12.5 46                                  2   370     320       3200     5.5   12.5 51                                  3   370     344       3200     5.8   12.5 46                                  4   370     474       3200     6.0   12.5 39                                  5   370     498       3200     5.5   12.5 39                                  6   370     500       3200     6.6   12.5 42                                  7   370     731       3200     5.2   12.5 44                                  8   372     751       3200     5.4   12.5 37                                  9   364      1        4000     3.4   13.3 24                                 10   368      3        4000     3.0   13.3 24                                 11   390      75       4000     2.27  13.3 24                                 12   390      99       3005     2.25  13.3 12                                 13   388     133       3005     3.12  13.3 12                                 14   390     153       3005     3.0   13.3 12                                 ______________________________________                                    

                  TABLE B                                                         ______________________________________                                                Selectivity                                                           Exp. Conver-  meth-    acetic                                                                              CO +  Methacrolein in                            No.  sion, %  acrolein acid  CO.sub.2                                                                            exit gas, mol %                            ______________________________________                                         1   44.6     89.6     3.4   4.2   2.2                                         2   36.8     89.8     2.5   3.3   1.82                                        3   41       89.3     3.1   3.9   2.13                                        4   48.8     85.3     3.9   5.5   2.5                                         5   61.5     90.8     1.66  2.9   3.07                                        6   41.3     88       3.9   2.6   2.4                                         7   31.4     88.8     2.0   4.3   1.45                                        8   33.5     90.2     2.6   3.9   1.63                                        9   51.6     86       3.5   7.5   1.51                                       10   60.4     89.4     3.6   4.6   1.62                                       11   73.7     89.6     2.0   7     1.5                                        12   69.4     89.6     1.4   7.8   1.4                                        13   81       85.8     4.3   7.9   2.17                                       14   83.1     84.6     4.4   8.8   2.11                                       ______________________________________                                    

EXAMPLE 2

Example 1 is repeated except that in making the catalyst composition theamount of thallium is doubled so that the atomic relationship's amongthe molybdenum, cobalt, iron, thallium, antimony, bismuth and siliconare 12, 4, 0.5, 2, 1, 1 and 8, respectively. The pertinent dataregarding the experiments using this catalyst for the oxidation ofisobutylene are set forth in the following Tables C and D. In theseexperiments the charge of catalyst amounted to 90 cc.

                  TABLE C                                                         ______________________________________                                        Exp. Temp.   Time,   Space Vel.                                                                             Feed, Vol. %                                    No.  ° C.                                                                           hr.     hr..sup.-1                                                                             C.sub.4 H.sub.8                                                                     O.sub.2                                                                             Steam                               ______________________________________                                        15   361     264     2055     6.8   13.6  30                                  16   376     288     2055     7.8   13.6  30                                  17   360     368     2055     7.0   13.6  28                                  18   360     392     2055     7.1   13.6  31.5                                19   370     416     2055     7.3   13.6  30                                  20   370     440     2055     7.4   13.6  31.5                                21   370     464     2055     7.5   13.6  30                                  ______________________________________                                    

                  TABLE D                                                         ______________________________________                                                Selectivity                                                           Exp. Conver-  meth-    acetic                                                                              CO +  Methacrolein in                            No.  sion, %  acrolein acid  CO.sub.2                                                                            exit gas, mol %                            ______________________________________                                        15   59.2     90.9     3.4   4.7   3.66                                       16   71.6     88.6     0.9   9.3   4.95                                       17   57.5     90.5     2.8   3.4   3.64                                       18   63.9     88.2     3.6   4.2   4.0                                        19   62.4     90       2.9   3.5   4.1                                        20   50.5     90.1     3.0   4.4   3.37                                       21   69.3     90.4     2.9   3.3   4.7                                        ______________________________________                                    

EXAMPLE 3

Example 1 is again repeated except that in making the catalystcomposition the silicon dioxide is omitted, the atomic relationshipsamong the other components remaining the same. As in Example 2, thecatalyst charge amounts to 90 cc. The data for the experiments usingthis catalyst are set forth in Tables E and F which follow.

                  TABLE E                                                         ______________________________________                                        Exp. Temp.   Time,   Space Vel.                                                                             Feed, Vol. %                                    No.  ° C.                                                                           hr.     hr..sup.-1                                                                             C.sub.4 H.sub.8                                                                     O.sub.2                                                                             Steam                               ______________________________________                                        22   366      5      2280     3.8   13.7  23                                  23   363      23     2290     3.5   13.7  21                                  24   369      95     2000     4.7   13.7  29                                  25   374     119     2000     5.1   13.7  29                                  26   380     143     2000     4.5   13.7  29                                  ______________________________________                                    

                  TABLE F                                                         ______________________________________                                                Selectivity                                                           Exp. Conver-  meth-    acetic                                                                              CO +   Methacrolein in                           No.  sion, %  acrolein acid  CO.sub.2                                                                             exit gas, mol %                           ______________________________________                                        22   89.9     80.5     8     8.5    2.75                                      23   82.1     80       8.2   8.6    2.3                                       24   88.5     81       7.7   8.5    3.37                                      25   94       75       8.2   10.1   3.6                                       26   95.2     75.4     7.0   11.3   3.23                                      ______________________________________                                    

It will, of course, be understood that the alcohol precursors of theolefins, e.g. compounds which by dehydration yield the olefin, such astertiary butyl alcohol in the case os isobutylene, can be used in placeof the specified olefin. In this case the dehydration of the precursorand the oxidation of the olefin take place in the reaction zone. Thefollowing example illustrates the use of tertiary butyl alcohol in placeof isobutylene to produce methacrolein by oxidation in the presence of acatalyst composition of the character described above.

EXAMPLE 4

Following the general procedure of Example 1 but using the catalystdescribed in Example 2 and employing tertiary butyl alcohol vaporsinstead of isobutylene in the feed, experiments were carried out for atotal of 283 hours. In each experiment the temperature was 360° C., thespace velocity was 2100 hr.⁻¹ and the composition of the feed gas was 11volume of tertiary butyl alcohol, 15 volume of oxygen and 20 volume ofsteam, the balance being nitrogen. The times of the experiments and theresults are set forth below in Table G. The conversion values arecalculated on the basis of isobutylene converted since it is consideredthat all of the tertiary butyl alcohol is dehydrated.

                                      TABLE G                                     __________________________________________________________________________                               Meth-                                                           Selectivity   acrolein                                                                            Isobutylene                                  Exp.                                                                             Time,                                                                             Conversion                                                                          meth-                                                                              acetic                                                                             CO +                                                                              in exit                                                                             in exit gas,                                 No.                                                                              hr. %     acrolein                                                                           acid CO.sub.2                                                                          gas, mol %                                                                          mol %                                        __________________________________________________________________________    1  208 37    88   1.7  4   3.45  6.6                                          2  233 36    88   1.7  5   2.9   6                                            3  238 38    88   1.45 5   3.2   6                                            4  260 45    90   1.7  3.8 4.4   6                                            5  261 40    90   1.46 3   3.6   6                                            6  263 39    90   1.5  3.4 3.4   6                                            7  266 38    87.5 1.36 6.2 3.1   5.7                                          8  270 37    87.5 1.74 3.8 3.4   6.7                                          9  278 42    90   1.57 2.2 3.9   6                                            10 281 37    87   1.47 3.3 3.5   6.8                                          11 283 34    90   1.42 2.3 3.2   6.8                                          __________________________________________________________________________

The embodiments of the invention in which an exclusive property isclaimed are defined as follows:
 1. A catalyst composition suitable forthe vapor-phase oxidation of an olefin to produce the correspondingunsaturated aldehyde with high selectivity comprising oxides ofmolybdenum, cobalt, iron, bismuth, thallium and antimony, and optionallysilicon.
 2. A catalyst composition as defined in claim 1, wherein themolybdenum, cobalt, iron, bismuth, thallium, antimony and silicon arepresent in the atomic ratios of 12, 0.2-8 0.05-5, 0.2-4, 0.05-5, 0.1-5and 0-20, respectively.